HSP101 functions as a specific translational regulatory protein whose activity is regulated by nutrient status

The 5′ leader (Ω) of tobacco mosaic viral RNA functions as a translational enhancer. Sequence analysis of a 102-kD protein, identified previously as a specific Ω RNA-binding protein, revealed homology to the HSP101/HSP104/ClpB family of heat shock proteins and its expression in yeast complemented a thermotolerance defect caused by a deletion of the HSP104 gene. Up to a 50-fold increase in the translation of Ω-luc, but not luc mRNA was observed in yeast expressing the tobacco HSP101 whereas Ω failed to enhance translation in the absence of HSP101. Therefore, HSP101 and Ω comprise a two-component translational regulatory mechanism that can be recapitulated in yeast. Analysis of HSP101 function in yeast translation mutants suggested that the initiation factor (eIF) 3 and specifically one (TIF4632) of the two eIF4G proteins were required for the HSP101-mediated enhancement. The RNA-binding and translational regulatory activities of HSP101 were inactive in respiring cells or in cells subject to nutrient limitation, but its thermotolerance function remained unaffected. This is the first identification of a protein required for specific translational enhancement of capped mRNAs, the first report of a translational regulatory function for any heat-shock protein, and the first functional distinction between the two eIF4G proteins present in eukaryotes.